The static-quadrupole-interaction frequency of the 8+ isomers in Ra-212 and Ra-214 and of the 2/17- isomer in Ra-213 has been measured with the level-mixing-spectroscopy (LEMS) method. Measurements have been performed both at low and high target temperatures (10 and 420 K) and on polycrystalline as well as single-crystalline targets. Shell-model calculations have been performed to demonstrate the influence of configuration mixing on the B(E2, 8+-->6+) values and the quadrupole moments of N = 126 isotones.